checking if the graph is bipartite.why is my function returning none?

Question

import collections
class Solution(object):
    def possibleBipartition(self, N, dislikes):
        graph = collections.defaultdict(list)
        for u, v in dislikes:
            graph[u].append(v)
            graph[v].append(u)

        color = {}
        def dfs(node, c = 0):
            if node in color:
                return color[node] == c
            color[node] = c
            for nei in graph[node]:
                dfs(nei,c^1)

        for node in range(1, N+1):
            if node not in color:
                dfs(node)
g=Solution()
N=3
dislikes=[[1,2],[2,3]]
print(g.possibleBipartition(N, dislikes))

Numerous solution is available online.But I am new to recursion. I want to understand why is my function returning none as the knowledge is going to help me later :) Thanks in advance.

Answer 1

Seems it's returning None since you don't have a return ... anywhere in your code. I'd make sure that you're explicitly returning where you need to be. For example, two spots that you might want to add a return statement:

return dfs(nei,c^1)

in your dfs() function as well as:

return dfs(node)

in your possibleBipartition() function.

Note that you need to explicitly specify the return in Python if there is one unlike in languages like Racket and Haskell.